<!DOCTYPE html>
<html>
<head>
	<title>满幅的背景，定宽的内容</title>
	<style type="text/css">
		footer {
			background: #ddd;
			height: 100px;
			/*padding: calc(50% - 450px);*/
		}
		.wrapper {
			height: inherit;
			background: #eee;
			max-width: 900px;
			margin: 1em calc(50% - 450px);
		}
	</style>
</head>
<body>
	<footer>
		<div class="wrapper">
			
		</div>
	</footer>

</body>
</html>